Semiconductor saturable absorbers are nonlinear optical elements that impose an intensity-dependent attenuation on a light beam incident upon it; an incident radiation of low intensity is preferably absorbed, while a high intensity radiation passes the saturable absorber with much less attenuation. For practicality, a semiconductor saturable absorber is usually integrated with a semiconductor, dielectric or metallic mirror forming a semiconductor saturable absorber mirror (SESAM). These devices have found applications in a large variety of fields. In particular, passive mode-locking based on semiconductor saturable absorber is a powerful technique to produce short optical pulses in simple laser cavities. Ultra short optical pulses have been produced with this technique using different SESAM designs. See for example the works published by F. X. Kärtner et al., IEEE J. Sel. Top. Quantum Electron., vol. 2, pp. 540-556, 1996, and B. C. Collings et al., IEEE J. Sel. Topics Quantum Electron, vol. 3, pp. 1065-1075, 1997 or U.S. Pat. No. 5,627,854 to Knox.
A SESAM comprises semiconductor material(s) whose energy band-gap is small enough to absorb an optical signal to be controlled, see for example U.S. Pat. No. 4,860,296 to Chemla et al. The absorbing material is usually embedded within semiconductor material(s) with a higher band-gap(s) that do not absorb the optical signal. The thickness of a single absorbing layer is typically in the range of few nanometers so that quantum-mechanical effects are enabled (in this case the absorbing layers are called quantum-wells, QWs). The whole absorber region may comprise a number of quantum-well layers representing the so-called multiple-quantum-wells structure. Additional design features can include positioning of the nonlinear absorbing layer within a Fabry-Perot cavity as well as means to apply an electrical field to the structure for the purpose of controlling its absorption properties, as shown by Heffernan et al. in Appl. Phys. Lett., vol. 58, pp. 2877-2879, 1991. Alternatively, an external optical source that provides a control beam can be used to vary the optical properties of the saturable absorber whereas the control beam can also be absorbed in the material surrounding the saturable absorber as for example is shown by M. Guina et al. in Opt. Lett., 28, pp. 43-45, 2003.
It can be gathered from the prior art that SESAMs are generally formed by utilizing compound semiconductor layers with similar lattice constants, i.e. small lattice mismatch, or the thickness of the lattice mismatched layers is kept below a critical thickness to ensure a high quality of the crystalline structure. The recovery time of high-quality lattice-matched SESAMs is in the nanoseconds range, as shown for example by Gray et al., Opt. Lett., vol. 21, pp. 207-209, 1996. However, for many applications, the saturable absorption should recover to its initial value in a much shorter time. In particular, for efficient and self-starting mode-locking, the recovery time should attain a value in the range of few picoseconds to few tens of ps, depending on the gain medium and laser cavity, as shown for example by R. Herda and O. G. Okhotnikov, Appl. Phys. Lett., vol. 86, pp. 01111-1-01111-3, 2005. To reduce the recovery time to suitable values, the fabrication process of SESAMs includes special techniques such as low-temperature growth, as shown by Gupta et al., IEEE J. Select. Topics Quantum Electron., vol. 10, pp. 2464-2472, 1992, Be-doping, for example shown by Qian et al., Appl. Phys. Lett., vol. 17, pp. 1513-1515, 1997, proton bombardment, see for example Gopinath, et al., Proceedings CLEO, 2001, pp. 698-700, and ion bombardment, as shown by Delponet al., Appl. Phys. Lett., vol. 72, pp. 759-761, 1998. Each of these techniques brings in different drawbacks, including an increased complexity of the fabrication process and may result in certain degradation of the SESAM parameters.
Combining semiconductor materials with large lattice mismatch, i.e. metamorphic structures, increases the degree of freedom in fabricating integrated semiconductor devices. In particular, for SESAMs it would be attractive to combine InxGa1-xAs absorbing regions, which are optically active at 1550 nm (x≧0.53), with high-quality and easy to fabricate GaAs-based distributed Bragg reflectors (DBRs). The lattice constants of these two material systems are significantly different giving raise to formation of defects that ultimately deteriorate the optical properties of the device. In order to reduce the amount of defects arising during the growth of such a structure, several approaches have been proposed.
According to a first approach, suggested by K. Weingarten et al. in U.S. Pat. No. 6,538,298 B1, a so called resonant design is employed to enhance nonlinear effects and thus enable to obtain a desired nonlinear effect by using very thin InxGa1-xAs active region absorbing 1550-nm radiation that is grown lattice-mismatched directly on GaAs-based DBR. The thickness of InxGa1-xAs should not exceed a critical thickness of about 5 nm. It was also suggested that by growing active regions that are 2 nm thicker than the relaxation thickness, certain number of dislocation defects are created leading to a reduction of the absorption recovery time. However, due to very thin active region, the nonlinear reflectivity cannot exceed 1.2-2.5%, a value that is too low for many applications.
According to a second approach, an InP buffer layer with a thickness of about 1-1.5 μm should be grown between the GaAs and the InGaAs active region, as for example demonstrated by A. G. Dentai et al., Electron. Lett. 22, 1186 (1986) or H. Q. Zheng et al., Appl. Phys. Lett., 77, pp. 869-871 (2000). This method has been used for the monolithic growth of 1550 nm SESAMs on GaAs substrate as presented by J. E. Cunningham et al. in U.S. Pat. No. 5,701,327. The aforementioned patent presents a standard fabrication method (included also in the previous references) employing multistep epitaxy for the growth of the InP buffer to limit the penetration of the dislocation defects formed at the GaAs/InP interface into the active region deposited on the top of the structure. A first part of the buffer is grown at lower temperature then the subsequent one, i.e. about 400° C., resulting in a confinement of the dislocations inside the first part of the buffer and thus avoiding their propagation into the upper layers. It was also suggested that the interface defects may act as recombination sources and thus decrease the recovery time of the absorption. Those skilled in the art would recognize that before being trapped by the interface defects the photocarriers generated within the active region would have to propagate towards the interface. This process impose certain limitation on the applicability of the method to control the recovery time of absorption; for example, if a high number of quantum-wells are employed the carriers generated at the proximity of the InP/GaAs interface will be preferentially trapped than those generated within the quantum-wells that are located far from the interface. One could expect that this method to reduce the recovery time is efficient only for a thin active region situated very close, i.e. within 50 nm, from the InP/GaAs interface.